1. Field of the Invention
The present invention relates to an extendable mast which can be deployed and folded. The extendable mast can be used as a component for such structures as space station and craft. It can also be used as a component for the extendable boom and the like.
2. Description of the Related Art
It is planned that the extendable mast is used as a structural member for a space station. This extendable mast which is folded to occupy a space as small as possible is mounted in the rocket or space shuttle and launched into the space. It is deployed in space and used as a structural member for structures such as a space station.
There are various types of the extendable masts and one of them includes a plurality of long rod-like longerons which can be freely bent. The extendable mast of this type is a truss-like structure made by connecting the longerons with a plurality of battens. It is folded by bending the freely-transformable longerons like coils. It is kept folded by a holder system. When it is to be deployed, it is released from the holder system. When it is released from the holder system, it is extended like a line due to the elasticity of the longerons to form a truss structure. The extendable mast of this type is simple in structure. However, its rigidity is low when it is extended.
This drawback is quite troublesome when the extendable mast of this type is used as a structural component for a large-sized space station. When it is used for this purpose, leaving its rigidity low, the natural frequency of the whole space station becomes low, thereby making it difficult for this frequency to be attenuated and controlled.
Another type of extendable mast is a truss-beam-like structure made by freely swingably connecting essentially-rigid members. The extendable mast of this joint type can be folded when these members are swung and folded. It is also extended when the members are deployed like a line. It can have a high rigidity but it becomes complicated in structure and this causes it to have a high weight.
The extendable mast of this joint type is disclosed in NASA (US) contractor Report 172461 (1986) "Batten Augmented Triangular Beam". A plurality of elastically-transformable battens are used in this case to form triangular frames. Two adjacent triangular frames of battens are connected to each other at their apexes by three longerons to thereby form truss-beam-like structure as a whole. Each of the longerons connecting the two adjacent triangular frames of battens consists of a pair of beam members, which are freely swingably connected by a pin. When the paired beam members are swung and folded, the extendable mast is also folded. When the mast is being folded, the battens are elastically transformed or deformed. Diagonals are stretched along diagonal lines of a rectangle which is formed by two battens adjacent in the vertical direction of the mast and by two longerons adjacent in the traverse direction of the mast. When the mast is extended, pull load is added to the diagonals and compression load to the longerons. In the case of this extendable mast, however, its rigidity is insufficient and its strength is low when it is extended, because the battens are elastically transformable.
Various types of extendable masts have been developed, but they could not sufficiently satisfy the purposes, counter to each other, of making them simple in structure, light in weight and high in their folding efficiency, and of making them high in rigidity when they are extended.